The present invention relates to magnetic signatures of marine vessels, more particularly to the control (e.g., limitation, reduction, or minimization) of magnetic signatures of ships through algorithmic logic that is computer-implemented in association with a shipboard degaussing system.
A marine vessel (e.g., a ship or submersible) produces a magnetic signature as the vessel moves in the Earth's magnetic field. The magnetic signatures characterizing the vessel may be detectable by enemy devices. For instance, a vessel's magnetic signature may render the ship susceptible to magnetic mines. Various methodologies have been implemented, especially in military contexts, to reduce this magnetic signature.
Many ships of the United States Navy include magnetic signature compensation systems, known as “degaussing systems,” which reduce ship magnetic signatures during ship motion in the Earth's magnetic field. The “closed-loop degaussing” (CLDG) as typically practiced by the U.S. Navy uses magnetometers, degaussing coils, and a computer (which executes a CLDG algorithm) to measure onboard magnetic fields, and to estimate offboard magnetic fields. Basically, CLDG actively compensates for the induced (nonpermanent) and permanent magnetic signals of a ship.
Nonpermanent magnetic signatures of ships in motion in a uniform magnetic field may be quantified, using quadrature analysis in the ship frame of reference, to produce two orthogonal signature components, viz., the “ferromagnetic induced” component and the “eddy current” component. The ferromagnetic induced component is in phase with the ambient magnetic field. The eddy current component is in phase with the time rate-of-change of this ambient magnetic field.
Some naval vessels, notably minesweepers, are constructed practically entirely from nonmagnetic material. Such vessels include magnetic signature control apparatus that assumes that the ferromagnetic induced signature is directly and linearly proportional to the ambient magnetic field, and that further assumes that the eddy current signature is directly and linearly proportional to the time rate-of-change of the ambient field. Practically speaking, these assumptions have been found to be correct for nonmagnetically hulled vessels (e.g., minesweepers), but incorrect for vessels (e.g., larger ships) that are constructed with greater amounts of magnetic and conductive material. Accordingly, for vessels that are more extensively conductive, these assumptions are erroneous and lead to inadequate signature compensation.
The following United States patents, incorporated herein by reference, are pertinent to degaussing of marine vessels: Schneider, “Closed-Loop Multi-Sensor Control System and Method,” U.S. Pat. No. 5,189,590, issued 23 Feb. 1993; Holmes et al., “Zero Field Degaussing System and Method,” U.S. Pat. No. 5,463,523, issued 31 Oct. 1995; Holmes et al., “Advanced Degaussing Coil System,” U.S. Pat. No. 5,483,410, issued 9 Jan. 1996; Mack et al., “Ship Degaussing System and Algorithm,” U.S. Pat. No. 6,965,505 B1, issued 15 Nov. 2005; Fitzpatrick et al., “High Temperature Superconducting Degaussing System.” U.S. Pat. No. 7,451,719 B1, issued 18 Nov. 2008.
The term “six degrees of freedom” is conventionally used to describe both translational motion and rotational motion of a body with respect to three perpendicular axes in three-dimensional space. In general, a seagoing ship is characterized by motion describable in terms of six degrees of freedom, viz., heave, surge, sway, roll, pitch, and yaw. The three kinds of translational ship motion are commonly referred to as heave (linear movement along a vertical axis), surge (linear movement along a horizontal fore-and-aft axis), and sway (linear movement along a horizontal port-and-starboard axis). The three kinds of rotational ship motion are commonly referred to as roll (rotational movement about a horizontal fore-and-aft axis), pitch (rotational movement about a horizontal port-and-starboard axis), and yaw (rotational movement about a vertical axis).